Exploring, drilling and completing hydrocarbon and other wells are generally complicated, time consuming and ultimately very expensive endeavors. As such, tremendous emphasis is often placed on well access in the hydrocarbon recovery industry. That is, access to a well at an oilfield for monitoring its condition and maintaining its proper health is of great importance. As described below, such access to the well is often provided by way of coiled tubing or slickline as well as other forms of well access lines.
Well access lines as noted may be configured to deliver interventional or monitoring tools downhole. In the case of coiled tubing and other tubular lines, fluid may also be accommodated through an interior thereof for a host of downhole applications. Coiled tubing is particularly well suited for being driven downhole, to depths of perhaps several thousand feet, by an injector at the surface of the oilfield. Thus, with these characteristics in mind, the coiled tubing will also generally be of sufficient strength and durability to withstand such applications. For example, the coiled tubing may be of stainless steel or other suitable metal based material.
In spite of being constructed of a relatively heavy metal based material, the coiled tubing is plastically deformed and wound about a drum to form a coiled tubing reel. Of course, a reel of slickline may also be provided in a similar fashion with a degree of plastifying deformation also occurring. Regardless, by making a reel of line available, these lines may be manageably delivered to the oilfield for use in a well thereat. In the case of slickline, once positioned at the oilfield, the line may be unwound from the reel and dropped vertically into the well to deliver tools coupled to the end thereof. In the case of coiled tubing, the tubing may be directed through the well by way of the noted injector equipment at the oilfield surface.
Unfortunately, due to the noted plastifying deformation which takes place during winding and unwinding of the above noted lines, yield strength is affected. That is, the amount of force necessary to achieve plastic deformation of a given line becomes less and less over time. So, for example, a coiled tubing that is rated 80,000 PSI in yield strength before initial use, may drop to a yield strength of 40,000 PSI after several dozen or so occurrences of winding and unwinding, or “cycling”, for applications at well sites.
Such repeated cycling as noted above leaves the line prone to cracking. So, for example, a threshold of a 40,000 PSI rating may be set, below which, a line may no longer be used. In this manner, the possibility of cracking may be avoided. In the case of slickline, such cracking could lead to breaking of the line, stranding downhole tools in the well along with a potentially significant amount of the line itself. In the case of coiled tubing, leaking may be a more likely occurrence.
In addition to cracking, coiled tubing faces the additional risk of ballooning, wherein internal hydraulic pressure of the tubing reaches a level that certain locations of the tubing are no longer able to withstand, leading to plastic diametral growth, or “ballooning”, and potentially bursting at such locations. That is, a reduction in the yield strength of the coiled tubing may be directly correlated with the likelihood of ballooning depending on the amount of differential pressure (e.g. hoop stress) that is imparted through the tubing. For example, the risk of ballooning is generally considered practical and likely when hoop stress exceeds 15% of yield strength. Thus, efforts are made to keep hoop stress below such a fairly predictable threshold as it corresponds to yield strength. This is in addition to refraining from coiled tubing use when the yield strength drops below a predetermined level (i.e. note the 40,000 PSI above).
Unfortunately, even though safe thresholds may be established for coiled tubing and slickline use, the ability to reliably stay below such thresholds is largely lacking. For example, depending on the specific make of a given line, modeling software may be used to generate a reference log which is able to predict remaining cycles to failure for the line over the course of its use. Thus, theoretically, the line may be tracked with each winding and unwinding. In turn, each subsequent user may then note the current cycle and reference the log to make sure that no thresholds are exceeded in relation to an application to be run. However, the use of such a reference log requires proper tracking of prior line deployments. That is, such tracking introduces the possibility of human error. Even simple mislabeling or incorrect entry of a serial number into the tracking system may render this technique unreliable.
Even where no human error is present for the technique described above, neither is any direct measurement. At best, this technique of predicting cycle life provides a user with a guess of what actual remaining life may be. Of course, this depends on the accuracy of the modeling software or the actual specifications of the line, which generally vary from the assumed specifications to a degree. Furthermore, even in the case of coiled tubing, where direct real-time measurement may be acquired by an integrity monitor which interfaces the line to check for ballooning, no yield strength data is provided. Thus, the possibility of cracking or emerging ballooning remains undetected. Ultimately, in order to avoid the exorbitant costs associated with line replacement at the well site, down time, and any necessary clean-up following line failure, the substantial costs of prematurely discarding coiled tubing and/or slickline are willingly incurred.